A REPORT ON THE 2012 SEISMIC SEQUENCE IN EMILIA (NORTHERN ITALY) Diego C. F. Lo Presti, & Mauro Sassu University of Pisa, Department of Civil Engineering, Largo Lucio Lazzarino, 1- Pisa, Italy, 56122, Italy, [email protected] , tel: +39 050 2217742, fax: +39 050 2217762 Lucia Luzi & Francesca Pacor INGV Sezione di Milano – Pavia, via Bassini 15, 20133 Milano, [email protected], tel. +39 02 23699280, fax +39 02 23699267 Doriano Castaldini & Giovanni Tosatti University of Modena and Reggio Emilia, Department of Chemical and Geological Sciences, Largo S. Eufemia 19, 41100 Modena, [email protected], tel. +39 059 2055856, fax +39 059 2055887 Claudia Meisina, Davide Zizioli & Francesco Zucca University of Pavia, Department of Earth & Environmental Sciences, via Ferrata 1 27100 Pavia, [email protected], tel. +39 0382 985831 Giuliano Rossi Provincia di Mantova, Progettazione della Viabilità e delle Infrastrutture, Via Principe Amedeo n. 32, Mantova [email protected], tel. ++390376 204285 Gilberto Saccorotti & Davide Piccinini INGV – Sezione di Pisa Via U. della Faggiola, 32 – 56126 PISA (I) tel +39 050 8311960 [email protected] ABSTRACT Since mid-May 2012, an energetic seismic sequence has affected the northern part of Italy and specifically a wide sector of the Po River Plain. The sequence has been dominated by two main events: a) Mw = 5.9 occurred near Finale Emilia on May 20th at a depth of 6.3 km, and b) Mw=5.8 occurred near Cavezzo on May 29th at a depth of 10.2 km (earthquake location are obtained Istituto Nazionale di Geofisica e Vulcanologia, http://iside.rm.ingv.it/). The effects of the two main shocks can be summarized as follows: - damage to infrastructures (roads, pipelines) essentially because of the occurrence of liquefaction phenomena or soil failure; - damage to very old constructions (especially churches and bell towers – masonry and brickworks); - recent constructions such as barns and industrial premises have collapsed. These constructions were mainly isostatic structures not designed to withstand earthquakes. The paper deals with the following arguments: - seismological aspects, mainly related to the seismo - tectonic framework, the source mechanisms, and the comparison between the observed seismic motion and that expected on the basis of the National Map of Seismic Hazard; - identification and mapping of the soil failures (liquefaction) and induced damage with special emphasis on the geomorphologic structures showing the presence of ancient riverbeds; - description of the structural damage regarding the historical buildings and the modern industrial buildings. In conclusion the report will try to explain the reasons for the large damage observed in the case of both ancient and modern constructions. INTRODUCTION Since mid-May May 2012, an energetic seismic sequence is May 20th at a depth of 6.3 km, and b) Mw = 5.8 occurred near interesting the northern part of Italy and specifically a wide Cavezzo on May 29th at a depth of 10.2 km. During the first sector of the River Po Plain pertaining to the Emilia Romagna two weeks of the sequence 7 earthquakes with magnitude Region. The sequence was preceded by a few foreshocks, the larger than 5 occurred. By the time of writing this note (early most energetic of which was a ML = 4.1 on May 19th, 2012. September, 2012) the catalogue from the national monitoring Thus far, the sequence has been dominated by two main system, ISIDe (http://iside.rm.ingv.it/) by the Istituto events, namely a) Mw = 5.9 occurred near Finale Emilia on Nazionale di Geofisica e Vulcanologia (INGV hereinafter), Paper No. EQ-3 1 accounts for more than 2,500 locations, with a completeness magnitude around 2. The effects of the two main shocks are summarized as follows: - 27 lives were lost, hundreds of persons were injured and at least 40,000 people were evacuated; - damage to infrastructures (roads, pipelines) essentially because of the occurrence of liquefaction phenomena or soil failures; - damage to very old constructions (especially churches, bell towers and masonry and brickwork); - as for the very recent constructions, many barns and industrial premises have shown a dramatic collapse. These constructions were mainly isostatic structures not designed to withstand earthquakes. In addition, relevant non - structural features were strongly damaged. Fig. 1. – Map of Northern Italy reporting the location of the - economic losses of some 2 billion euros. strongest (Mw > 5) earthquakes of the 2012 sequence (red A special issue on the preliminary data and results of the stars). The two mainshocks have attached the focal solution, Emilia seismic sequence was published by Anzidei et al. and are labeled by the corresponding origin time and moment (2012). magnitude. Main cities are labeled. In particular, the following topics are dealt with:- seismological aspects referring mainly to the focal The seismic sequence and historical seismicity mechanisms and the level of the seismic motion in comparison with the prescription of the Italian Building Code; The seismic sequence began on 2012 May 20th (02:03:53 - identification and mapping of the soil failures UTC), with a Mw 5.86 (ML 5.9) earthquake. This mainshock (liquefaction) and induced damage, with special emphasis on was preceded, a few hours before, by a Mw 3.98 (ML 4.1) the geomorphologic structures showing the presence of foreshock, almost co-located with the main event. Within the ancient riverbeds; following 15 days, the seismic sequence included six - description of the structural damages regarding the additional earthquakes with magnitude greater than 5.0 (see historical building and the modern industrial buildings. Fig. 1). The most energetic of these, located about 12 km west of the May 20th mainshock, was a Mw 5.66 (ML 5.8) on May 29th (07:00:03 UTC). By the time of writing this paper (early SEISMOLOGICAL ASPECTS September, 2012), the sequence is gradually waning (Fig.2). The two most energetic events, similarly to other large shocks Tectonics of the area of the sequence, exhibit a focal mechanism consistent with the activation of EW-striking thrust faults, as indicated by time The seismic sequence object of this paper developed along the domain moment tensor solutions (TDMT) routinely calculated Northern Apennines frontal thrust system, which is constituted using optimal subsets of INGV's national seismic network by a pile of NE-verging tectonic units that formed as a (Scognamiglio et al., 2009; Fig. 1), and in agreement with the consequence of Cenozoic collision between the European and tectonic setting and active stress field of the area. Preliminary the Adria plates (Boccaletti et al. 2011; see Fig. 1). This thrust analyses of the May 20th mainshock (Piccinini et al., 2012) front is covered by a thick (less than 100 meters to 8 km) Plio- indicate that this event was constituted by at least three Pleistocenic sedimentary deposits of fluvial origin. distinct sub-events, likely related to multi-lateral rupture Nonetheless, geometry and location of the main faults are now propagation toward the eastern and western tips of the fault well established, thanks to extensive seismic reflection plane. The May 29th, Mw=5.8 event occurred about 12 km surveys conducted during the 70's for oil and gas exploration WSW of the May 20th mainshock activating an adjacent fault (Ori and Friend, 1984). Present-day, active NE-SW segment. At present, the geometrical relationships between the compression throughout the outer Apennines front and Po structures activated by these two main events are still to be plain is documented by GPS data, that show an average clarified. horizontal shortening of approximately 1 mm/y (Zerbini et al., 2006), and by both borehole breakouts data (Montone et al., Epicenters extend approximately 50 km along the EW direction; the hypocentral depths span the 0-40 km depth 2004) and centroid moment tensor solutions of large (Mw > 4) interval, with the large majority of hypocenters concentrated earthquakes (Pondrelli et al. 2006). within the shallowest 10 km of crust. Although these catalog locations do not allow for a clear identification of the geometries of the activated structures, a NS cross-section Paper No. EQ-3 2 delineates a plane dipping South by about 45° (Fig. 3, Both historical (Rovida et al., 2011) and instrumental bottom), which is consistent with one of the focal planes catalogues (ISIDE database: http://iside.rm.ingv.it) indicate obtained from moment tensor inversion of the most energetic that the seismicity rate for this sector of the Po Plain is lower shocks (Fig 1) and with the inferred setting of the thrust front. than that characterizing the Northern Apennines belt. The These data thus confirm that the current activity of Northern most important historical event is the Mw = 5.5 earthquake Apennines thrust systems is controlled by an overall North – which, on late 1570, struck the city of Ferrara. This shock was South oriented compressive stress field acting on EW-striking, followed by a complex series of aftershocks, lasting till early S-dipping buried thrust faults. 1572. For the 1570 main-shock, historical reports document liquefaction effects (Galli, 2000). Response Spectra and seismic response analyses The strong motion data have been obtained from the permanent and temporary stations of the Italian strong motion network managed by the Department of Civil Protection (DPC, www.protezionecivile.gov.it). Additional data have been obtained from the network managed by INGV. Table 1 Strong motion stations and parameters of the Italian Building Code NTC (2008) for the response spectra: STA = station code; LA_ST = station latitude; LO_ST = station longitude; EC8 = EC8 class; Network = recording network (IT Fig. 2 – Temporal evolution of sequence, in terms of = DPC); the two series of values ag F0 and T*c are the Magnitude (top) and daily number of earthquakes (bottom) parameters of NTC (2008) for return periods of 475 and 975 versus time.